1. Field of the Invention
The present invention relates to an armature structure for driving a printing wire and a dot head used in a wire dot printer.
2. Description of the Related Art
The wire dot printer is arranged to move a printing wire (hereinafter, simply referred to as a wire) called a needle forward and backward to strike the tip end of the wire against a print medium thereby to print a dot-shaped image thereon. Since the wire dot printer employs such the printing method, the wire dot printer can simultaneously print plural slips etc. in a stacked state and so is employed for business use. Although there are various kinds of methods as the printing method of moving the wire (needle) forward and backward, the method called a clapper type is generally employed. The clapper type has been employed widely since the structure thereof is simple and a relatively long stroke can be secured. Such the kind of the printing method is proposed by JP-A-2005-75000, for example.
The dot head of such the clapper type includes a plurality of armature structures each for driving a corresponding printing wire backward and forward. The armature structure is configured in a manner that an arm having a printing needle attached to a tip end thereof is integrated with an armature which is driven magnetically. Each of the arm and the armature is provided at the base end thereof with a through hole into which a fulcrum shaft is inserted. The arm rotates around the fulcrum shaft when the armature is attracted magnetically, whereby the tip end of a needle provided at the tip end of the arm collides with a print medium to perform the printing.
In order to realize a high-speed printing, the armature structure is required to be light-weighted and have a high magnetic efficiency. To this end, the armature is formed by iron-cobalt alloy which is excellent in magnetic characteristics. The iron-cobalt alloy is excellent in magnetic characteristics but has a nature that it is hard and fragile. In the case of manufacturing the armature, the punching processing using the press processing is utilized thereby to form the armature in a predetermined shape. In this case, when the hard and fragile iron-cobalt alloy is subject to the punching processing using the press processing, a shear plane A and a broken-out section B are formed on the pressed end face in the thickness direction thereof as shown in FIG. 1.
For example, when the armature with a thickness of 0.80 mm is subjected to the punching processing, a ratio between the shear plane A and the broken-out section B becomes almost 50% (that is, each of the shear plane and the broken-out section becomes 0.40 mm). Of course, when a clearance between a male die and a female die for the punching processing is made small, a ratio of the shear plane A can be made large. However, since the material to be punched is hard, the punching dies may be broken when the clearance is made too small. It is difficult to increase the ratio of the share plane to almost 50% or more in view of the productivity.
The aforesaid shear plane A and the broken-out section B appear in the thickness direction also at the inner periphery of the through hole in which the fulcrum shaft of the armature fits. The armature structure rotates at a high printing frequency of 2500 Hz around the fulcrum shaft at the time of the printing operation. Thus, the inner periphery of the through hole of the armature is worn away due to the contact with the fulcrum shaft. An amount of the abrasion differs between the hear plane A and the broken-out section B, so that there arises the eccentric wear and so the armature structure inclines. Due to the inclination of the armature structure, the needle attached to the tip end of the arm is applied with a stress in the transverse direction, so that the durability performance of the arm is badly influenced by the stress.